The present invention relates to open top refrigerated display cases having an ambient air defrost system. Both within the specification and the claims of the present application, all references to refrigeration apparatus or refrigeration operations are intended to include cooling both at a temperature below 32.degree. F., such as associated with frozen food display cases, and in excess of 32.degree. F., such as typically associated with dairy food and fresh meat display cases.
In the operation of all types of refrigerated display cases, it is desirable to include a system capable of automatically defrosting the display case. The defrost cycle can be actuated either at set periodic times or when the frost buildup within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. By this manner of operation, it is possible to avoid any significant frost buildup within the display case.
Typically within the prior art, there have been three different approaches employed for defrosting refrigerated display cases. The first approach involves total reliance on the use of electric resistance heaters that are arranged adjacent to the refrigerated coils of the refrigeration mechanism. During a defrost cycle, these heaters supply heat in an effort to eliminate the frost buildup on the coils. The particular technique is relatively simple both in its construction and operation. However, since the electrical heaters are high voltage heaters that utilize significant electricity during operation, with the rapidly increasing cost of electricity it has become extremely uneconomical to employ such systems as the primary and sole source of heat for defrosting the coils.
A second type of system circulates hot compressed gaseous refrigerant through the refrigeration coils during the defrost cycle. During the defrost cycle, a valve control mechanism shuts off the supply of refrigerant to the refrigeration coils and alternatively feeds superheated compressed gaseous refrigerant through the coils. This hot gas serves to melt away any frost buildup that has accumulated on the refrigeration coils but simultaneously provides heat within the air conduit which can be circulated through the display case, which is disadvantageous. While this type of system does not suffer from the high cost of operation of the electrical heater defrost system, the use of heated gas involves a relatively high construction cost. Due to the requirement that the system be able to selectively switch between the supply of heated gas and refrigerant to the refrigeration coils, a complicated valving structure must be provided. Such a mechanism significantly increases the cost of construction of the display case. In addition, the provision of such a complicated system only increases the number of complex parts capable of breaking down and necessitating costly repairs.
A third type of system employed for defrosting display cases relies upon ambient air. It is this general category with which the invention of the present application is concerned. One type of system that employs ambient air during the defrost cycle is exemplified by those embodiments illustrated in U.S. Pat. Nos. 3,404,525, 3,850,003 and 3,937,033 all to Beckwith et al. Each of these systems uses fans separate from the main air circulating fans for circulating air during the defrost cycle of operation. These extra fans are turned on during the defrost cycle for pulling ambient air from outside of the display case into the air conduits. A second type of system is illustrated in U.S. Pat. No. 3,082,612 to Beckwith, which system draws ambient air into the main air circulation path through ports located in the lower front part of the refrigerated display case. Such ports are normally closed during the refrigeration cycle and are open during the defrost cycle. The Beckwith et al. '003 patent indicates that the concepts described in U.S. Pat. Nos. 3,082,612 and 3,403,525 did not prove to be practical and hence were not commercially feasible.
Finally a third type of air defrosting system is shown in U.S. Pat. No. 4,144,720 to Subera et al. which is assigned to the same assignee as the present application. In the foregoing patent, an open front refrigerated display case having primary and secondary air conduits is disclosed. In this system, reversible fans are employed for reversing the direction of flow of air within the conduits and simultaneously drawing in air from outside of the display case.
Another system employing reversible fans for ambient air defrost is shown in U.S. Pat. No. 4,026,121 to Aokage et al. This patent, however, refers to short-circuiting the air flow between the primary and secondary air bands for the purpose of supplying warmer air to the primary band.
It has been recognized that an ambient air defrost operation can be incorporated into an open top refrigerated display case as disclosed in German Offenlegungsschrift No. 21 23 646 and U.S. Pat. Nos. 4,120,174 to Johnston and 4,182,130 to Ljung. This German patent illustrates an open top refrigerated display space having an ambient air defrost system. While during the defrost cycle of operation the direction of the air flow is reversed, the quantity and velocity of such flow remain unchanged. The patent to Ljung illustrates an island display case in which a tube arranged within the central wall extends above the display case for drawing in ambient air during the defrost cycle of operation. The direction of the air flow during the defrost cycle of operation is the same as the direction during the refrigeration cycle of operation and the air continues to be omitted out of the same outlet openings.
The Johnston patent illustrates an open top display case having a single air conduit extending around the case. During the refrigeration cycle, the air flows in a first direction and during the defrost cycle the direction of air flow is reversed with ambient air being drawn into the conduit. The quantity of air flow during the defrost cycle is greater than the refrigeration air flow. The defrost air, after passing through the conduit, is expelled in a direction up and over the access opening in the top of the refrigerated display case. It has been found during the development of the present invention that with a defrost air flow pattern such as disclosed in the Johnston patent that a significant portion of the expelled air will fall back towards the access opening in the refrigerated case and reenter the air conduit at the other side of the access opening. The existance of such an air flow pattern has been confirmed by brochures released by Kysor Industrial Corporation, the assignee of the Johnston patent wherein in the island display cases the air flow patterns illustrated show that the defrost air reenters the air conduit and hence is recirculated through such conduits. The air flow patterns illustrated in the Kysor brochures are shown in FIGS. 1 and 2 of the present application, which figures are labeled as prior art.
When ambient air surrounding the refrigerated display case is drawn in and utilized as a source of thermal energy in order to defrost the refrigeration coils, the ambient air after passing through the frosted coils can be at a low temperature, particularly during the initial portion of a defrost cycle. In such a condition, the ambient air does not cause thermal shock to the stored products if it should come in contact therewith during a defrost cycle of operation. However, during the terminal portion of the defrost cycle the defrost ambient air is elevated to a temperature above the temperature of the air mass in the display section of the case that surrounds the stored products and hence this defrost air will cause thermal shock to the products when coming into contact with the products. This problem is particularly acute in the defrosting of open top cases with ambient air.
One approach to alleviating the problem caused by the defrost ambient air coming into contact with the stored refrigerated products is to construct the case or cabinet with guide means which during the defrost cycle of operation directs the defrost air upwardly so as to lie over the access opening of the case. German Offenlegungsschrift No. 28 04 008, published Aug. 3, 1978 shows a guide plate 29 in FIG. 1 which directs defrost air upward from the normal flow conduit in order to expell the defrost air to the space above the stored products. This defrost air being cooler than the ambient air tends to settle back toward the display space. The defrost air then will come into contact with the products in the display space and also will be drawn back into the air conduit and recirculated. This case suffers from the same problems of the case disclosed in the patent to Johnston.
Another air guide device is set forth in Australian Patent Specification No. 51774/79 where an inverted U-shaped channel is used to guide ambient defrost air away from the stored refrigerated products and downwardly toward the outside of the display cabinet. The operation of this system leaves the display space unprotected from the ambient air above the case. The contact of the ambient air with the air in the display space may result in thermal shock to the products within the case.
Another solution to the above stated problem is set forth in U.K. Patent Application Ser. No. 20166 69A which illustrates an open front display case. Here the changing thermal and volumetric differences between the defrost ambient air band and the air mass surrounding the stored products are utilized together with a means to create a negative pressure in the display space. The defrost air is first permitted into the display space and is thereafter caused to be directed away from the display space without the provision of special guide means.
U.S. Pat. No. 3,324,783, to Hickox, assigned to the same assignee as the present invention, discloses a restricting slot 18 in an air conduit for a different purpose. In that patent, the restriction slot causes a back pressure which results in a uniform distribution of the propelled air along the longitudinal dimension of the merchandise cabinet so that the air flow is uniform prior to passing through an air directing grid structure. The purpose of the grid structure is to allow the refrigerated air band to flow more uniformly across the open top of the cabinet. There is no provision in this patent for expelling an air band away from the cabinet.